Integrin superfamily proteins are heterodimeric cell surface receptors, composed of an alpha and beta subunit. At least 18 alpha and 8 beta subunits have been reported, which have been demonstrated to form 24 distinct alpha/beta heterodimers. Each chain comprises a large extracellular domain (>640 amino acids for the beta subunit, >940 amino acids for the alpha subunit), with a transmembrane spanning region of around 20 amino acids per chain, and generally a short cytoplasmic tail of 30-50 amino acids per chain. Different integrins have been shown to participate in a plethora of cellular biologies, including cell adhesion to the extracellular matrix, cell-cell interactions, and effects on cell migration, proliferation, differentiation and survival (Barczyk et al, Cell and Tissue Research, 2010, 339, 269).
Integrin receptors interact with binding proteins via short protein-protein binding interfaces. The integrin family can be grouped into sub-families that share similar binding recognition motifs in such ligands. A major subfamily is the RGD-integrins, which recognise ligands that contain an RGD (arginine-glycine-aspartic acid) motif within their protein sequence. There are 8 integrins in this sub-family, namely αvβ1, αvβ3, αvβ5, αvβ6, αvβ8, αIIbβ3, α5β1, α8β1, where nomenclature demonstrates that αvβ1, αvβ3, αvβ5, αvβ6, & αvβ8 share a common αv subunit with a divergent β1 subunit, and αvβ1, α5β1 & α8β1 share a common β1 subunit with a divergent a subunit. The β1 subunit has been shown to pair with 11 different α subunits, of which only the 3 listed above commonly recognise the RGD peptide motif (Humphries et al, Journal of Cell Science, 2006, 119, 3901).
The 8 RGD-binding integrins have different binding affinities and specificities for different RGD-containing ligands. Ligands include proteins such as fibronectin, vitronectin, osteopontin, and the latency associated peptides (LAPs) of Transforming Growth Factor β1 and β3 (TGFβ1 and TGFβ3). Integrin binding to the LAPs of TGFβ1 and TGFβ3 has been shown in several systems to enable activation of the TGFβ1 and TGFβ3 biological activities, and subsequent TGFβ-driven biologies (Worthington et al, Trends in Biochemical Sciences, 2011, 36, 47). The diversity of such ligands, coupled with expression patterns of RGD-binding integrins, generates multiple opportunities for disease intervention. Such diseases include fibrotic diseases (Margadant et al, EMBO reports, 2010, 11, 97), inflammatory disorders, cancer (Desgrosellier et al, Nature Reviews Cancer, 2010, 10, 9), restenosis, and other diseases with an angiogenic component (Weis et al, Cold Spring. Harb. Perspect Med. 2011, 1, a 006478).
A significant number of αv integrin antagonists (Goodman et al, Trends in Pharmacological Sciences, 2012, 33, 405) have been disclosed in the literature including inhibitory antibodies, peptides and small molecules. For antibodies these include the pan-αv antagonists Intetumumab and Abituzumab (Gras, Drugs of the Future, 2015, 40, 97), the selective αvβ3 antagonist Etaracizumab, and the selective αvβ6 antagonist STX-100. Cilengitide is a cyclic peptide antagonist that inhibits both αvβ3 and αvβ5 and SB-267268 is an example of a compound (Wilkinson-Berka et al, Invest. Ophthalmol. Vis. Sci, 2006, 47, 1600), that inhibits both αvβ3 and αvβ5. Invention of compounds to act as antagonists of differing combinations of αv integrins enables novel agents to be generated tailored for specific disease indications.
Pulmonary fibrosis represents the end stage of several interstitial lung diseases, including the idiopathic interstitial pneumonias, and is characterised by the excessive deposition of extracellular matrix within the pulmonary interstitium. Among the idiopathic interstitial pneumonias, idiopathic pulmonary fibrosis (IPF) represents the commonest and most fatal condition with a typical survival of 3 to 5 years following diagnosis. Fibrosis in IPF is generally progressive, refractory to current pharmacological intervention and inexorably leads to respiratory failure due to obliteration of functional alveolar units. IPF affects approximately 500,000 people in the USA and Europe.
There are in vitro experimental, animal and IPF patient immunohistochemistry data to support a key role for the epithelially restricted integrin, αvβ6, in the activation of TGFβ1. Expression of this integrin is low in normal epithelial tissues and is significantly up-regulated in injured and inflamed epithelia including the activated epithelium in IPF. Targeting this integrin, therefore, reduces the theoretical possibility of interfering with wider TGFβ homeostatic roles. Partial inhibition of the αvβ6 integrin by antibody blockade has been shown to prevent pulmonary fibrosis without exacerbating inflammation (Horan G S et al Partial inhibition of integrin αvβ6 prevents pulmonary fibrosis without exacerbating inflammation. Am J Respir Crit Care Med 2008 177 56-65). Outside of pulmonary fibrosis, αvβ6 is also considered an important promoter of fibrotic disease of other organs, including liver and kidney (Reviewed in Henderson N C et al Integrin-mediated regulation of TGFβ in Fibrosis, Biochimica et Biophysica Acta—Molecular Basis of Disease 2013 1832.891-896), suggesting that an αvβ6 antagonist could be effective in treating fibrotic diseases in multiple organs.
Consistent with the observation that several RGD-binding integrins can bind to, and activate, TGFβ, different αv integrins have recently been implicated in fibrotic disease (Henderson N C et al Targeting of αv integrin identifies a core molecular pathway that regulates fibrosis in several organs Nature Medicine 2013 Vol 19, Number 12: 1617-1627; Sarrazy V et al Integrins αvβ5 and αvβ3 promote latent TGF-β1 activation by human cardiac fibroblast contraction Cardiovasc Res 2014 102:407-417; Minagawa S et al Selective targeting of TGF-β activation to treat fibroinflammatory airway disease Sci Transl Med 2014 Vol 6, Issue 241: 1-14; Reed N I et al. The αvβ1 integrin plays a critical in vivo role in tissue fibrosis Sci Transl Med 2015 Vol 7, Issue 288: 1-8). Therefore inhibitors against specific members of the RGD binding integrin families, or with specific selectivity fingerprints within the RGD binding integrin family, may be effective in treating fibrotic diseases in multiple organs.
SAR relationships of a series of integrin antagonists against αvβ3 αvβ5, αvβ6 and αvβ8 have been described (Macdonald, S J F et al. Structure activity relationships of ac integrin antagonists for pulmonary fibrosis by variation in aryl substituents. ACS Med Chem Lett 2014, 5, 1207-1212. 19 Sep. 2014).
It is desirable to provide αvβ6 antagonists which may also have activities against other αv integrins, such as αvβ1, αvβ5 or αvβ8.